For years it has been common practice to cover metallurgical vessels such as pot furnaces and arc furnaces towards the top for reasons of industrial safety and environmental protection. Additionally, it has been common practice to connect such vessels to a device for suctioning off the gasses and dusts developed during processing. Such a device is known, e.g., from Techn Mitt. Krupp, Volume 18 (1960), No. 3 at page 111 and also from West German published patent document DE-PS No. 28 48 948.
On the other hand, molten metals with especially high degrees of purity are increasingly in demand. This means, e.g., for steel metals adhering to extremely small contents of oxygen and/or nitrogen. Such contents can be achieved only if the steel melt during the pot treatment is shielded from the ambient air as completely as possible (e.g., by use of protective gas or by processing in vacuum). For the production of steels with extremely low oxygen content, a device has been proposed in EPC Patent Application No. 00 92 652 (corresponding to U.S. Pat. No. 4,401,464) which essentially consists of a pot and a suction hood provided with a heat shield. The heat shield covers the pot virtually completely. However, a continuous air gap is left between the rim of the pot and the heat shield. The suction hood extends downwardly beyond the air gap. The outer wall of the suction hood forms a continuous opening with the wall of the vessel through which ambient air can be sucked in by means of a connected suction device. The suction hood and/or the heat shield has one or several openings through which in each case a lance can be inserted into the interior of the pot. The gasses and dusts escaping from the melt during the pot treatment can overflow into the interior of the suction hood between the heat shield and the pot rim through the air gap and are removed with the sucked-in ambient air.
Decisive for the operability of such an installation, aside from adequate dimensioning of the suction device, is especially the size and time constancy of the effective intake cross-section for the process gasses and the ambient air. In this respect the device according to EPC Patent Application No. 00 92 652 (corresponding to U.S. Pat. No. 4,401,464) has the disadvantage that, during operation, skulls may form on the upper rim of the pot, thereby preventing the heat shield from being lowered to the required height above the pot. As a result, the air gap between the pot rim and the heat shield is enlarged, so that the flow rate in the air gap and thus the suction effect is reduced. Since the ambient air that is sucked in also passes through this air gap, it can happen that, e.g., because of an outer air flow running crosswise to the pot, on one side air is pressed in through the air gap into the interior of the pot, while on the opposite side a correspondingly greater amount of gas is suctioned off from the interior of the pot. Thus, despite the use of suction from the pot and the use of protective gas for feeding in the substances required for pot treatment and a certain resulting excess pressure in the interior of the pot, it is not guaranteed that the melt is shielded against oxygen and nitrogen from the air. As a makeshift, to avoid the side effects, customarily doughnut-shaped packing seals filled with refractory materials are placed on the pot rim to achieve sealing.
It is a further disadvantage of this device that, in feeding in the substances for pot treatment, the melt starts bubbling, and gases and dusts escape from the melt which are entrained by the always present strong gas flows (warm air currents) directed upwardly. As a result, despite the pot suction, at least part of the gases and dust press outwardly, especially past the centrally placed lance, through the openings in the heat shield or in the suction hood. The consequence of this uncontrolled thermal flow also is that air flows inwardly against the desired direction of flow through the annular gap between the pot and the lid and there causes a build-up and oxygen contamination of the steel.